This invention relates generally to the field of deposition of silicon by chemical vapor deposition, and more particularly to methods for Heating a Fluidized Bed Silicon Deposition Apparatus which are more convenient, more efficient, of lower cost and provide better quality silicon beads than existing methods
High purity polycrystalline silicon is the basic raw material of the semiconductor and photovoltaic industries. It is currently produced by the decomposition of highly purified silicon containing gases onto a heated high purity silicon surface. This process is termed chemical vapor deposition. The standard industry technique uses high purity silicon rods as the heated surface. An alternative fluidized bed technology is also used on a limited scale. This latter technology provides a large surface area of heated silicon on the surface of beads fluidized by the silicon bearing gas and other diluents and offers the promise of reduced capital and operating costs and production of a more convenient form of silicon in the shape of beads. Many attempts have been made to implement the fluidized bed technology but all have suffered from severe operational and purity problems, which have prevented full commercialization.
The major use for the polycrystalline silicon is in production of single crystal silicon via melting and growth of single crystal silicon boules in Czochralski crystal pullers. Such pullers have specific requirements with regard to feeding the beads, contamination, ease of melting etc. which must be met in order to use silicon beads. Kajimoto et al documents some of these issues in U.S. Pat. No. 5,037,503.
The major operational problem is sintering of the beads in the reactor and the resultant plugging of the reactor, the major purity problems are metals, carbon, oxygen and hydrogen in the bulk and surface of the product and the major problem in feeding beads to the crystal puller is difficulty in controlling the bead flow due to variation in shape and size.
The sintering appears to be more prevalent as the temperature, deposition rate, silicon containing gas concentration and bead size increases and less prevalent as the fluidizing gas flow rate increases. Hence a violently fluidized bed will have a lower tendency to sinter but may tend to blow over more dust and will require more heat.
It has been accepted that it is important to have a reactor that does not contaminate the product and that the use of metal reactors is not feasible, see Ling U.S. Pat. No. 3,012,861 and Ingle U.S. Pat. No. 4,416,913 and hence metal contamination can be resolved by not contacting the beads with any metal parts.
Similarly contact with carbon or carbon containing materials leads to carbon contamination so graphite or silicon carbide parts are usually coated with silicon. Oxygen normally comes in through oxygen containing compounds such as water, carbon monoxide and carbon dioxide in the inlet gases and hence all such compounds are removed from the gas streams to as great a degree as is practicable. Oxygen containing materials such as silicon oxide (quartz) are frequently used as containment materials, see Ingle above, and can be used in contact with silicon although care must be taken to prevent erosion.
Hydrogen contamination is primarily caused during the deposition process when hydrogen remains trapped in the bead. This is a time, temperature and deposition rate dependent process which has been described by A. M. Beers et al xe2x80x9cCVD Silicon Structures Formed by Amorphous and Crystalline Growth,xe2x80x9d Journal of Crystal Growth, 64. (1983) 563-571. For rapid deposition rates of the order of 2-3 8 micron/minute which are desired in commercial reactors the silicon surface temperature must exceed 800 C. Typical rod reactors usually operate above this temperature as do halosilane based fluid bed reactors and thus such reactors do not suffer from this problem. The current silane based commercial fluid bed reactors operate below this temperature in at least part of the reactor and consequently have dusting problems see Gautreaux and Allen U.S. Pat. No. 4,784,840 and require a second dehydrogenation step as described by Allen in U.S. Pat. No. 5,242,671.
The problem of size and shape is not as important but most polycrystalline consumers would prefer large round beads because they flow better and have less surface area, thus less risk of contamination. Large beads require more gas flow to fluidize and hence more heat to bring said gas up to operating temperature.
One standard way to heat a fluidized bed is through the walls because the heat transfer from the wall to the particles is very good and wall heaters can be easily and cheaply built using electric heating coils. Another standard way is to preheat the gas reactants. A further standard approach is to recover heat from both the solid and gaseous effluent of the reactor by means of heat exchange. A yet further standard approach is to recycle unused reactant and or carrier gas. In a silicon deposition reactor there are problems facing all of these approaches. If the wall is heated then it is by definition hotter than the bed particles and hence more likely to be deposited on as the reaction rate is strongly influenced by temperature. The rule of thumb is that reaction rate doubles with each 10 degree Celsius rise in temperature. Hence a hot wall causes wall deposits which are a loss of product, increase the resistance to heat transfer through the wall and can cause breakage of the reactor on cool-down due to differential thermal expansion. There is also the problem that the heat load is localized to the inlet area where the incoming gases are heated up to reaction temperature. Thus hot beads may be present in the reactor but unable to circulate down to the inlet zone fast enough to provide sufficient heat. Heating the gas reactants is restricted by the thermal decomposition of the silicon bearing gases at around 350-400 C. Thus the gases cannot be heated above this temperature without depositing in the heater or in the inlet to the reactor. This problem is further compounded by heat conducted back into the inlet from hot beads located just above the inlet of the silicon bearing gases. The surface temperature of these beads should be over 800 C. to prevent hydrogen contamination, hence there is a high temperature gradient between the beads at 800 C. and the inlet which needs to be below the thermal decomposition temperature of the silicon containing gases which is 350-400 C. Recovery of heat is difficult because of the tendency of the silicon containing gas to form wall deposits which in turn means the wall temperature must be below 350 C. which is difficult when cooling gases or solids which are at 800 C. or greater. Recycle of unused reactants or carrier gas is also difficult for the same decomposition reason. The recycle gas must be cooled to below the thermal decomposition temperature of the silicon containing gases before mixing with them.
Thus the prior technology has attempted to deal with the heating issue in a variety of ways. Ingle, U.S. Pat. No. 4,416,913 noted the use of microwaves to heat the silicon beads directly through the quartz wall which itself is not heated by microwaves. Poong et al. in U.S. Pat. No. 4,900,411 advises using microwaves and notes the need to cool the wall and the distributor grid in order to prevent silicon deposits, which can then absorb the microwaves. Iya in U.S. Pat. No. 4,818,495 also suggests cooling the distributor grid and providing a heating zone above the reacting zone to compensate. Kim et al in U.S. Pat. No. 5,374,413 notes that cooling of the wall is not effective in preventing wall deposits and greatly increases power consumption and suggests a partition between the reacting and heating zone. Partitions have also been suggested by Ingle see above and Van Slooten in U.S. Pat. No. 4,992,245. Neither Iya in U.S. Pat. No. 4,818,495 nor Van Slooten in U.S. Pat. No. 4,992,245 provided means for the heated beads to travel down to the reacting area in sufficient quantity to heat the incoming gases and offset the distributor cooling. Lord in U.S. Pat. No. 5,798,137 suggests use of xe2x80x9cjet heatingxe2x80x9d where lasers are used to heat through the inlet jet itself or chlorine is added to react with silane in the jet region. Lord in U.S. Pat. No. 5,810,934, also suggests using an isolation tube between the inner tube containing the silicon containing gases and the outer tube containing the hot beads in order to control the wall temperature of the inner tube below the decomposition temperature. This suffers from the two disadvantages of reducing the heat transfer and the heat transfer rate. Hence only a portion of the available heat can be recovered thus requiring additional bead cooling and the surface area must be larger than would be required other wise. Lord in fact recognizes this and provides an alternate approach using a water cooled bead cooler.
All the prior technology makes provision for dilution of the silicon bearing gas before the mixture is fed to the reactor stream and so the inlet gas temperature is still limited by the decomposition temperature of the silicon bearing gas which is typically around 350 C. Kim and Van Slooten also make provision for a separate entry for a carrier gas into the heating zone which is separated from the reaction zone by a partition and they claim this gas may be heated up to the reaction temperature although in their examples the actual temperature, is below that. In Van Slooten""s example the inlet gas is 500 C. compared to reactor temperatures of 650 C. at the top and 550 C. and a heating zone temperature of 660 C. In Kim""s examples the carrier gas preheat temperature was 250 C. and 350 C.
The prior technology had difficulty in reaching the required high temperatures, greater than 800 C., without contaminating the product or plugging the reactor. These high temperatures are needed, particularly at the gas inlet, for production of hydrogen and dust free product. A critical deficiency of the prior technology, with the exception of Lord in U.S. Pat. Nos. 5,798,137 and 5,810,934, is the failure to recognize the importance of the need to maintain high temperatures according to the experimental data and theoretical relationships in the article of A. M. Beers et al xe2x80x9cCVD Silicon Structures Formed by Amorphous and Crystalline Growth,xe2x80x9d Journal of Crystal Growth, 64. (1983) 563-571. This article details the relationship of temperature, time and deposition rate with higher deposition rates requiring higher temperatures and times in order to crystallize the deposited amorphous silicon and release the codeposited hydrogen.
In the prior technology the inlet area has the most serious problems in product quality because of a combination of factors all of which tend to prevent the needed crystallization to produce polycrystalline silicon and remove hydrogen and/or other codeposited elements such as halogens. This area has the highest silicon bearing gas concentrations, the lowest temperatures and the least post deposit time for the beads. The deposit rates tend to be highest at the inlet because of the high silicon containing gas concentrations and the rapid decomposition of the silicon bearing gases once the temperature is above 500 C. The temperatures are lower because the incoming gases are cold and cool the beads near the inlet as the gases warm up. Finally the beads are removed at or near the bottom of the reactor which is also the inlet for the gases thus the beads removed have just been deposited on and hence have little time to crystallize the recent deposits and dehydrogenate. Of these factors the most important one is the temperature because the crystallization is strongly dependent on temperature. Frequently the prior technology aggravates this problem by cooling the distributor grid. Thus in the prior technology most of the reaction and deposition occurs near the inlet and much of this deposit is unsuitable because of its powdery nature and high hydrogen content. Iya in U.S. Pat. No. 4,818,495 shows a temperature profile where the zone just above the grid is at 500 C. and the top of the bed is at 770 C. Hence the product would be very dusty and contaminated with hydrogen. Similarly in Van Slooten U.S. Pat. No. 4,992,245 the distributor surface is cooled to a temperature between 200-400 C. and he states in his example that the temperature at the top of the fluidized bed is 923 K (700 C.) and at the bottom is 823 K (600 C.). Again the product would be dusty and contaminated with hydrogen. Kim has the reactive gas distributor cooled to 318 C. in his example 2 and has a CVD reaction temperature of 930 C. Since the partition isolates the reaction zone from the heating zone and is half the bed height the beads next to the reactive gas inlet are much colder than the upper part of the reactor as the hot beads from the heater section do not mix with them. These beads are primarily heated through the quartz partition which itself is deposited on by the silicon containing gases in the reaction zone. This silicon wall deposit will be hotter than the beads in the reaction zone and will thus grow at a more rapid rate.
This problem of wall deposit on the partition is also faced by the reactor described in the Van Slooten U.S. Pat. No. 4,992,245. It is apparent that the provision of a partition does not avoid the problem of wall deposits it merely relocates them to the partition. Thus the requirement for a partition is an additional deficiency in the prior technology.
The provision of a partition can help the bead quality if the beads are removed from the heating zone of the partitioned reactor since the beads have more time at a higher temperature without any deposition. Unfortunately such post deposition crystallization and dehydrogenation suffers from the problem that the hydrogen must diffuse out through the complete deposit thickness and this can take several hours or days as shown by Allen in U.S. Pat. 5,242,671.
This amount of time is usually not available as a practical matter since it requires a significantly larger reactor and also higher temperatures (1000-1100 C.).
Lord in U.S. Pat. No. 5,798,137 recognizes the need to remove hydrogen as the deposition occurs in order to minimize the distance the hydrogen has to diffuse out and provides localized xe2x80x9cjet heatingxe2x80x9d at the inlet with lasers and or chlorine. The major deficiencies of this approach are that laser heating is expensive and inefficient and the equipment is high maintenance and chlorine heating is expensive, reduces yield and contributes contaminants.
The provision of a partition requires a carrier gas to fluidize the beads on the heating side of the partition. Since this is not a reactive gas it can be heated above the decomposition temperature of the silicon bearing gas and both Kim and Van Slooten claim this feature in their patents. However the sensible heat of the carrier gas is not used to directly heat the reacting beads and neither Van Slooten nor Kim claim the possibility of heating the carrier gas above the reaction temperature. In the example by Kim the carrier gas is 4.0 mole/min of hydrogen at 250 C. and the reactive gas is 3.1 mole/min of trichlorosilane and 6.0 mol/min of hydrogen at 100 C. Neither temperature is above the decomposition temperature of TCS (350 C.) or remotely close to the stated CVD reaction temperature of 930 C. In fact more hydrogen is used as a diluent in the reactive gas than is used as xe2x80x9cheatedxe2x80x9d carrier gas.
The CVD reaction temperature of 930 C. is low for trichlorosilane deposition by the hydrogen reduction reaction and will result in lower yield of silicon as is shown in example 1 where the TCS feed is 0.35 mol/min and the silicon deposition rate is 1104 grams over ten hours which calculates to 1.85 grams/min or 0.066 mol/min. This is a yield of 18.8% of the silicon in the TCS. The preferred temperature for hydrogen reduction is above 1000 C. and preferably 1100-1250 C. as noted in Padovani U.S. Pat. No. 4,207,360. Obtaining such temperatures required use of high temperature materials such as silicon carbide coated graphite walls which could operate significantly hotter than the beads. Unfortunately this approach causes carbon contamination of the silicon making it unusable. The source of the contamination is primarily a reversible gas phase reaction;
SiC+2H2=Si+CH4
Silicon Carbide+Hydrogen=Silicon+Methane The methane gas is formed at the silicon carbide walls and mixes in with the silicon beads where it decomposes to form silicon carbide thus contaminating the beads. At such high temperatures the silicon carbide diffuses rapidly through the silicon wall deposit to continually replenish the surface.
Thus the prior technology suffers from an inability to obtain the desired high temperatures in the inlet region and/or required for high silicon yield without forming severe reactor or partition wall deposits, plugging the inlet or distribution means, resorting to expensive, exotic and unreliable heating means or contaminating the product.
A further deficiency of most of the prior technology is its failure to provide sufficient post deposition time at temperature to complete the crystallization and dehydrogenation of the product needed to produce low dust and hydrogen content silicon beads.
A primary object of the invention is to provide methods of heating a fluidized bed silicon deposition reactor, which can provide high reactor temperatures through out the reactor and especially near the inlet or inlets of the silicon containing gases without plugging said inlet or inlets
Another object of the invention is to provide methods of heating which supplement established methods of heating through the wall or jet.
Another object of the invention is to provide methods of heating which can be used separately or in combination.
A further object of the invention is to provide methods, which will improve the reactor deposition efficiency.
Yet another object of the invention is to provide methods which will improve reactor energy efficiency and operability.
Still yet another object of the invention is to provide methods which will improve product appearance and quality.
Methods for Heating a Fluidized Bed Silicon Deposition Apparatus comprising the steps of one or more entries to the reactor for the solids or gases which can be heated without decomposition separate from one or more entries for the gas or gases which decompose to form silicon when heated, heating the solids or gases which do not decompose to temperatures between 400-2000 C., more preferably 800-1600 C., separately heating the gases which do decompose thermally to temperatures less than the temperature at which they decompose, typically 25-400 C., preferably 300-350 C., and providing means for the solids and gases which do not decompose to be removed from the reactor and for some of their heat content to be recovered and reused in the reactor.
Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.